The present invention relates in general to communication systems, and is particularly directed to the use of a sub-rate integrated services digital network (ISDN) channel for reducing the vulnerability of Dataphone Digital Service (DDS) subscriber lines to impairments such as bridge taps, and to also extend the effective operating range of DDS subscriber lines to distances (on the order of 22 Kft) well beyond those currently possible at T-carrier time slot channel data rates (56 and 64 Kbps).
The currently defined American National Standards Institute (ANSI) standard T1.601 governing 2B1Q modulation, two-wire, full-duplex data transfer with echo cancellation requires that ISDN basic rate digital subscriber lines (having a data rate of 144 kbps, with bidirectional data payload, plus overhead maintenance channels), must not exceed a two-wire loop loss of 42 dB at 40 KHz, for 1300 ohms, resistive. This requirement effectively limits the operational range of a conventional copper cable link (No.26 (American Wire Gauge) AWG wire) driven by commercially available ISDN transceiver equipment to a distance on the order of 15 Kft.
The U.S. Pat. No. 5,598,413, to M. Sansom et al, (hereinafter referred to as the ""413 patent), issued Jan. 28, 1997, entitled: xe2x80x9cFour Wire, Half-Rate Architecture with Embedded Differential Delay Compensation for Extending Range of Basic Rate ISDN Communications,xe2x80x9d assigned to the assignee of the present application and the disclosure of which is herein incorporated successfully addresses this limited range problem by a one-half rate, four-wire, ISDN demultiplexingxe2x80x94multiplexing architecture, that uses an out-of-band maintenance channel for the transport of differential delay compensation information. Advantageously, this scheme is able to extend the normal range of ISDN basic rate digital subscriber lines to a distance on the order of 25 Kft, which is well beyond those currently possible using a repeaterless two-wire transmission path.
My co-pending U.S. patent application Ser. No. 08/744,975, filed Nov. 7, 1996, entitled: xe2x80x9cQuarter-Rate 2B1Q Architecture with Embedded Differential Delay Compensation for Extending Range of DDS Communications,xe2x80x9d assigned to the assignee of the present application and the disclosure of which is herein incorporated (hereinafter referred to as the ""975 application) solves a similar range limitation problem for four-wire DDS communications, by using a standard ISDN transceiver chip to demultiplex quarter-rate (2B1Q) ISDN channels for the transport of DDS data over a four-wire transmission path between an office channel unit data port (OCU DP) and a customer premises site. As in the scheme described in the ""413 patent, by operating at a frequency that is one-quarter of the frequency associated with the ISDN transceivers, the reduced data rate of the four-wire system provides a trade-off against loop loss, increasing the distance over which DDS may be provided, without further stipulation or constraint upon the requirement of loops considered as DDS candidates in terms of loop loss and bridge tap.
Fortunately, because well over 95% of two-wire copper cable links currently installed in the United States do not exceed this (15 Kft) distance, the above described range limitations for the transport of ISDN and DDS channels are not yet a significant problem in most installations. However, because the physical layout of a typical copper link contains one or more bridge taps that are distributed along its length, and many of which are unterminated, the typical copper link is vulnerable to interference.
In accordance with the present invention, advantage is taken of the ability to drive an ISDN transceiver chip at a reduced clock rate for the transport of DDS signals, which not only serves to mitigate the effect of potential interference sources such as bridge taps, but is also effective to extend the transport range of a DDS channel (to a distance on the order of 22 Kft). In accordance with the sub-rate DDS channel transport network architecture of the present invention, each of the ISDN transceiver unit of an OCU DP at one end of a two-wire path and the ISDN transceiver unit of a DDS termination unit at an opposite end of the two-wire path is driven at only a reduced clock ratexe2x80x94such as one-half the normal clock ratexe2x80x94which not only reduces the operating frequency for rates of 56 kbps and 64 kbps, but mitigates against the impairing effects of interference sources such as bridge taps. It also provides a trade-off against loop loss, and thereby enables the effective range of the two-wire DDS circuit to be geographically extended (up to a distance on the order of 22 kft), while complying with applicable industry standards for loop deployment and testing.
As will be described, at an OCU DP site, a time division multiplexed data stream of a T1-carrier system is coupled to timeslot interface circuitry of the OCU DP. For each time slot of the incoming T1 data stream, the timeslot interface circuitry couples 64 Kbps data bytes to the digital communication port of a sub-rate clocked ISDN transceiver unit. By clocking the ISDN transceiver U-chip at a reduced fraction of the standard ISDN basic rate frequency, such as one-half rate as a non-limiting example, the ISDN transceiver unit outputs a reduced frequency, sub-rate 2B1Q channel (such as a 80 KHz half-rate 2B1Q channel containing two 32 Kb/s bearer channels, and a 16 Kb/s overhead channel), that provides extended range functionality and reduced susceptibility to interference. This sub-rate ISDN channel is transported from the transceiver unit of the OCU DP over the two-wire pair to the ISDN transceiver at the DDS termination unit of a customer premises site. At the DDS termination site, a DDS AMI four-wire interface outputs a standard DDS AMI signal onto a four-wire link for delivery to customer premises equipment.
Since the network architecture of the invention does not employ separate, demultiplexed data paths, it does not require disassembly and reassembly of the 64 Kb/s data stream over multiple transport paths, thereby obviating the need for differential delay compensation circuitry, and thus reducing the complexity of each of the OCU DP and the DDS termination unit. For a DDS 64 Kb/s channel, a half-rate (80 KHz) operation of each ISDN transceiver U-chip also allows for the transport of a 16 Kb/s signalling channel.